The present invention relates to a method for operating furnace equipment for magnesium alloys in which a bath surface is covered with a protective furnace gas containing a sulfur compound.
In installations where liquid magnesium is present it is necessary to take special precautions to reliably prevent the access of atmospheric oxygen because magnesium is extremely combustible in the liquid state. SF6 is used as such a protective furnace gas, for example, which reacts with the magnesium of the melt and forms a protective layer which protects the melt from the access of atmospheric oxygen. Occasionally, mixtures of SF6 and nitrogen are used for this purpose. Since SF6 concerns a greenhouse gas, its use is problematic for reasons of environmental protection. Moreover, the costs for this gas are relatively high.
It has already been considered as an alternative to use SO2 as a protective furnace gas. SO2 is available at relatively low cost and, like SF6, forms a protective layer on a magnesium melt. The application of SO2 has been prevented up until now because it concerns an extremely unpleasant smelling gas which in medium to high concentrations is detrimental to health and promotes corrosion. During the use of SO2 in the conventional manner it is not possible to prevent the escape of gas from the furnaces or the like which leads to an impermissible burden on the ambient environment. In particular, maximum workplace concentrations will be exceeded by far.
It is further known to use gas mixtures of SO2 and dried air as a protective furnace gas for magnesium furnaces. It has not yet been managed, on the one hand, to control the toxic and corrosive effect of SO2 and, on the other hand, to realise a secure operation of the furnace.
It is the object of the present invention to provide a method which, on the one hand, allows a secure treatment of magnesium melts and, on the other hand, causes the lowest possible burden on the environment.
This object is achieved in accordance with the invention in such a way that a mixture of SO2 and an inert gas is used as a protective furnace gas, with the volume share of SO2 being set precisely to a value which is in a range of between 0.3% and 1.5%, preferably between 0.5% and 1.0%. It was surprisingly noticed that in the case of SO2, there is a small range in which there is a sufficient protective effect on the one hand and a burden on the environment can be substantially prevented on the other hand. The relevant aspect of the present invention is the setting of a precise value of the SO2 concentration in the protective furnace gas. Since very low SO2 concentrations are used, precise process control and the avoidance of fluctuations in the gas composition is very important to prevent fires.
Nitrogen is used particularly preferably as the inert gas. Nitrogen is available at low cost and is not critical to the environment. In a particularly preferable embodiment of the method in accordance with the invention, it is provided that the mixture of SO2 and the inert gas is performed in an air-conditioned room whose temperature is kept above 22xc2x0 C., and preferably in a range of between 25xc2x0 C. and 35xc2x0 C. It has been noticed that a satisfactory mixture of SO2 and nitrogen is only possible from a certain minimum temperature at a performance pressure  greater than 2 bars. Moreover, temperature-induced fluctuations in volume and pressure of the involved gases can be securely prevented by the mixture in an air-conditioned room. A temperature of approx. 30xc2x0 C. is particularly preferable.
Moreover, the present invention relates to an apparatus for producing a protective furnace gas for furnace equipment for magnesium alloys with a storage tank for a sulfur compound, a storage tank for an inert gas and a mixing device. The apparatus in accordance with the invention is characterized in that the mixing device is arranged to provide a precise flow rate control of SO2 and nitrogen and is arranged in an air-conditioned room. Safety gas cells are provided in an air-conditioned room which receive the SO2 cylinders. Nitrogen is taken from a conventional tank. The mixture of SO2 and nitrogen is performed through electronic mass flow meters which are arranged in the air-conditioned room. The gas mixture thus produced is conveyed to the respective furnace equipment via pipelines and adjusted to consumption by way of local control systems. The furnace equipment can concern smelting furnaces, holding furnaces, dosing furnaces and pig casting belts for magnesium alloys.
SO2 sensors are provided in the safety gas cells which already respond to low SO2 concentrations. Once an SO2 cylinder has been emptied, the same is scavenged with nitrogen together with the pipelines in order to exclude any health hazards during the exchange of the cylinders. In this manner it can be prevented reliably that any SO2 escapes into the environment.
As a result of the low SO2 concentration of the protective furnace gas, any pollution of the environment and any annoyance caused by bad smell, particularly in the furnace area, can be securely prevented. Secure operations can be ensured by the precisely set mixture ratio.